The present invention relates to a technology to reduce dependence of an arrayed waveguide grating (AWG) on temperature. In particular, the present invention relates to a technology in which by removing temperature dependence of a wavelength characteristic of an arrayed waveguide grating, external temperature compensation by a Peltier device or the like which has been indispensable to attain stable multiplexing and demultiplexing characteristics in an optical communication system in the prior art can be dispensed with.
Prior Art
In an optical fiber communication system, when dense wavelength division multiplexing (DWDM) is employed to increase transmission capacity, a multiplexer and/or demultiplexer (optical filter) is quite important to multiplex and to demultiplex wavelengths.
Among various optical filters, an arrayed waveguide grating has a narrow-bandwidth wavelength characteristic and a high isolation ratio. The grating is a filter device of multiple input and multiple output type. Therefore, the filter device can demultiplex multiplexed signals and can multiplex demultiplexed signals. Using the optical filter, it is possible to easily configure a optical filter device.
When such a device includes a silica-based (or silica) waveguide, the device can efficiently coupled with an optical fiber so that the device operates with a low insertion loss of about several decibels (dB). The device consequently has attracted attention and has been intensively discussed and studied in the world.
FIG. 11 shows a general configuration of an arrayed waveguide grating of the prior art. In general, the grating includes at least one input waveguide 7, an input-side slab waveguide 8, arrayed waveguides 9 which are several tens to one hundred and several tens in number, output-side slab waveguide 10, and a plurality of output waveguides 11 as shown by FIG. 10.
In operation, multiplexed light incident to the input waveguides 7 spreads or expands in the input slab waveguide 8 to enter the arrayed waveguides 9, the light of each waveguides 9 having an identical phase (co-phase). The light in the respective arrayed waveguides 9 differ in incident light intensity from each other. The intensity shows substantially a Gaussian distribution.
The arrayed waveguides 9 differ in optical path length from each other. The difference is fixed so that the optical path sequentially increases or decreases in the waveguides 9. Therefore, light beams respectively propagating through the waveguides 9 arrive at the output slab waveguide 10 with a fixed phase difference therebetween. Due to chromanic dispersion, a cophasal surface inclines depending on the wavelength.
As a result, light beams focus at different positions depending on the wavelengths on an interface between the output slab waveguide 10 and the output waveguides 11. Therefore, by arranging an output waveguide at each focusing position, light having a desired wavelength can be attained from the output waveguide.
In the arrayed waveguide grating, the optical path difference is an essential parameter for wavelength selectivity as above. However, in the optical waveguide of the prior art, the optical path length depends on temperature. This leads to a problem of variation in a filter transmission bandwidth (central wavelength) depending on ambient temperature. For example, when the waveguide is made of silica-based glass, the central wavelength of the optical filter varies about 0.01 nanometer (nm; about 1.3 gigaherz (GHz) in notation of central frequency) at about every 1 degree due to the temperature dependence of the optical path length. In a device using a semiconductor-based material, the temperature dependence is about ten times the dependence above.
To remove the problem of temperature dependence, a high-precision temperature controller is added to the device in a known method. This however increases the system cost and the size of the device, and reliability of the overall system is lowered.
To remove the temperature dependence of the arrayed waveguide grating of the prior art, there has been proposed a method to cancel the temperature dependence of the optical path length difference in a arrayed waveguide section. The optical path length difference depends on temperature because of temperature dependence of a refractive index of the optical waveguide material and expansion coefficient or contraction of material of a substrate due to temperature change.
In the arrayed waveguide grating, the temperature dependence of the optical path length difference in the arrayed waveguide section is canceled, for example, as described by H. Tanobe et. al. in pages 235 to 237 of the “IEEE Photon. Technol. Lett.”, Vol. 10, No. 2 1998. According to the article, waveguides are fabricated respectively with two different kinds of InP-based materials respectively having different values of the refractive index temperature coefficient to thereby disposing a waveguide section having a high refractive index temperature coefficient value and a waveguide section having a low refractive index temperature coefficient value. By adjusting lengths of the respective sections, the temperature dependence of optical path length is canceled in the arrayed waveguide section.
However, it is difficult to manufacture, in one plane, waveguides respectively with different materials (particularly, in a planar lightwave circuit). This leads to a problem of a complex and long production process. In the above-mentioned prior art, the optical waveguide is not constructed in the form of a planar lightwave circuit. The system includes a thin clad layer on a two-dimensional planar core layer and a lib is formed thereon. The structure is attended with a problem that light confinement is insufficient and radiation loss increases by bending or flexing the waveguide. The arrayed waveguide grating of the prior art has a total loss of 16 dB to 18 dB.
Like the article above, a relatively simple production process using two different kinds of materials having different refractive index temperature coefficient values has been described in pages 1945 to 1947 “Electron. Lett.”, Vol. 33, No. 23, 1997 written by Y. Inoue et. al. As can be seen from FIG. 12, in arrayed waveguides 12 (a part of the arrayed waveguides 9 shown in FIG. 11), a groove 13 having a contour of a wedge is formed to reach a substrate. The groove 13 is filled with silicone 14 with a negative value of refractive index temperature coefficient to cancel with a positive value of the silica-based waveguide section. The temperature dependence of optical path length difference is thereby solved between the arrays.
In FIG. 12, assume that the silica-based waveguide section 12 has an equivalent refractive index of nsio2, the silicone 14 has an equivalent refractive index of nsilicone, the waveguide length difference between adjacent waveguides of silica-based arrayed waveguide section is ΔLsio2, and the waveguide length difference between adjacent waveguides of silicone section is ΔLsilicone. To remove the temperature dependence of optical path length in the arrayed waveguide section,                                                         ⅆ                                                                                   ⅆ              T                                |                                    n                              SiO                2                                      ⁢            Δ            ⁢                                                   ⁢                          L                              SiO                2                                              |                      +                                          ⅆ                                                                                               ⅆ                T                                              |                                    n              Silicone                        ⁢            Δ            ⁢                                                   ⁢                          L              Silicone                                |                =        0                            (        1        )            must be satisfied.
The example leads to problems as below. The groove disposed in the waveguides is only filled with silicone, namely, light incident to the groove is not confined in the groove. Therefore, the light expands and spreads in the horizontal and vertical directions. That is, all of the light having propagated through the groove does not enter again in the silica-based waveguide section and hence the loss is increased (about 2 dB loss is increased in the example of the prior art). Additionally, since the value of loss depends on the groove length, the inherent power distribution of each arrayed waveguide is shifted in the silica-based waveguide section after the groove.
In the prior art, to remove the temperature dependence of the arrayed waveguide grating, the production process is extremely complex, the device insertion loss is increased, and/or the light power in the arrayed waveguides shifts from the inherent power distribution.